The present invention relates to a focus yoke, an electromagnetically focused CRT display with the focus yoke, and a negative feedback circuit with stray-capacitor-cancellation means used for the CRT display or the like.
Electromagnetically focused CRTs heretofore include two types, one being solenoidal current type and the other being cylindrical permanent magnet type (hereinafter simply referred to as permanent magnet type).
A typical CRT and peripheral components thereof are shown in FIG. 12. In FIG. 12, a reference numeral 1 denotes a CRT, 2 a deflection yoke (hereinafter referred to as DY), 3 a focus yoke (hereinafter referred to as FY), 4 an electron gun, 5 a neck portion.
The focus yoke 3 functions as an electron lens for converging electron beams as represented by arrows in FIG. 12.
A sectional structure of a solenoid current type focus yoke and the distribution of the magnetic field along the axis of the neck portion 5 of this type of the CRT are shown in FIGS. 13A and 13B, respectively.
A sectional structure of a permanent magnet type of focus yoke and the distribution of the magnetic field along the axis of the neck portion 5 are shown in FIGS. 14A and 14B, respectively.
In FIGS. 13A and 14A, a reference numeral 6 denotes a magnet path forming magnetic core comprising a soft magnetic material, 7 a winding, 8 a permanent magnet. Marks in circles of the winding 7 denote the direction of currents. Arrows in the neck portion 5 denotes the direction of magnetic field. A curve 9 in FIG. 13B denotes the magnetic field distribution in the solenoid current type CRT and a curve (solid line) 10 and a curve (dotted line) 11 in FIG. 13B denote the magnetic field distributions in the permanent magnet type, respectively.
FIGS. 13A and 14A correspond to FIGS. 13B and 14B in positional relation in an axial direction of the CRT, respectively.
In the permanent magnet type CRT of FIG. 14A, the winding 7 is provided for so-called dynamic focussing, that is for dynamically changing a focussing force depending upon the position on a screen.
Problems of the above mentioned prior art are listed in FIG. 15. These problems will be described.
Focussing performance of each of the above mentioned focus yokes will be described. In the permanent magnet type CRT, a magnetic field of an inverse polarity (magnetic field components having a desired direction and the opposite direction) is inevitably generated as is apparent from the curve 11 in FIG. 14B. The magnetic component of the inverse polarity will cause spherical aberration in an electromagnetic lens. Accordingly, the permanent magnetic type CRT has a problem that the focussing performance on a screen deteriorates. The solenoid current type CRT has excellent focussing performance since no magnetic field of the inverse polarity is generated so that the spherical aberration is less as is apparent from the curve 9 in FIG. 13B.
The solenoid current type focus yoke shown in FIG. 13A is excessively heavy while the permanent magnet type focus yoke shown in FIG. 4A is light.
The solenoid current type focus yoke consumes very much electric power for dynamic focussing and the permanent magnet type focus yoke also consumes much electric power.
In the focus yoke shown in FIG. 13A, the winding 7 comprises two sets of windings which are independent of each other. That is, the focus yoke shown in FIG. 13A comprises a first set of windings (D.C. and low frequency windings) which are externally disposed and a second set of windings (high frequency horizontal scanning windings) which are internally disposed. The focus yoke shown in FIG. 14A also comprises two sets of windings which are independent of each. That is, the focus yoke shown in FIG. 14A comprises a first set of windings (low frequency windings) which are externally disposed and a second set of windings (high frequency windings) which are internally disposed.
The reasons why the diameter of the core 6 is so large that a useless space is formed therein in the structure shown in FIG. 14A are as follows: That is, reduction in the space in the core 6 by decreasing the diameter of the core 6 is effective to reduce the wastfull magnetic energy and the consumed electric power. On the other hand, decreasing of the diameter of the core 6 will enlarge the curve 11 in FIG. 14B in the inverse polarity direction to provide a remarkably large spherical aberration.